Wireless mobile devices such as mobile phones have become increasingly multifunctional, and there is demand for their size to be smaller. A reduction in the size and thickness of the devices is therefore required. High frequency filters used in mobile phone devices are generally a SAW filter that employs surface acoustic waves (SAW), an FBAR filter that employs the piezoelectric oscillation of a thin film, or the like. Features of resonators and filters that employ elastic waves include being small and inexpensive. Elastic wave elements are therefore essential to reducing the size of communication devices such as mobile phones.
Examples of elastic wave elements include a surface acoustic wave element and an elastic boundary wave element that include comb-shaped electrodes that are formed on a piezoelectric substrate, piezoelectric thin film, or the like from an alloy whose main component is aluminum, an alloy whose main component is copper, or the like, and that have a period corresponding to a desired frequency, and such elements employ elastic waves that are excited by the comb-shaped electrodes. Also, examples of elastic wave elements include a resonator, a filter, and a duplexer. For example, in the case of a 1 terminal pair (1 port) resonator, a dual resonance characteristic of having a resonance frequency and an antiresonance frequency is exhibited. Employing this characteristic has enabled the practical use of ladder filters in which 1 port resonators that have different comb-shaped electrode periods are connected in series and in parallel to form a ladder shape. Other examples include a DMS (Double mode SAW) filter in which a resonator is formed by a plurality of comb-shaped electrodes, and an IIDT filter that has an excitation comb-shaped electrode and a reception comb-shaped electrode.
Surface acoustic wave elements employ waves that travel across the surface, and therefore space is necessary on the surface of surface acoustic wave elements (e.g., see Patent Document 1 and Patent Document 2). For this reason, a package in which the surface has space thereon is necessary, which has been a hindrance to a reduction in the thickness of the overall device. In view of this, elastic boundary wave elements have been developed as elastic wave elements in which space on the surface is not necessary (e.g., see Patent Document 3). Patent Document 3 discloses an elastic boundary wave element that has a piezoelectric material layer made from a piezoelectric substrate, a piezoelectric thin film, or the like, at least two types of dielectric layers, namely a first dielectric layer and a second dielectric layer, on the piezoelectric material layer, and furthermore a comb-shaped electrode that has been formed in the vicinity of the surface of the piezoelectric material layer on the first dielectric layer side and whose period λ corresponds to a desired frequency. With the elastic boundary wave element disclosed in Patent Document 3, the peaks of the wave energy are in the first dielectric layer.
Note that Patent Document 4 is a technical document relating to the present invention.                Patent Document 1: Japanese Laid-open Patent Publication No. 05-63500        Patent Document 2: U.S. Pat. No. 5,694,096        Patent Document 3: International Publication Pamphlet No. WO 98/52279        Patent Document 4: Japanese Laid-open Patent Publication No. 2005-117641        
With a structure in which boundary waves are almost entirely confined within the first dielectric layer, as in the elastic boundary wave element disclosed in Patent Document 3, it has been found that there is generation of boundary waves having a frequency different from the desired frequency and having a maximum amplitude on the first dielectric layer side in the vicinity of the boundary between the second dielectric layer and the first dielectric layer (hereinafter, also called “unnecessary waves 1”). Also, as waves that are likewise unnecessary, it has been found that there is generation of unnecessary waves due to the leakage of displacement up to the surface of the elastic boundary wave element (i.e., the surface of the second dielectric layer) (hereinafter, also called “unnecessary waves 2”).
The above-described unnecessary waves 2 are surface waves, and can be suppressed by providing an acoustic absorption unit on the surface of the elastic boundary wave element or providing a scatter pattern on the surface of the second dielectric layer of the elastic boundary wave element. However, the above-described unnecessary waves 1 are boundary waves, and cannot be suppressed by providing an acoustic absorption unit or the like.